1. Field of the Invention
Embodiments of the present invention generally relate to the color calibration of color rendering devices and more specifically, to a method and system for generating color profiles to be applied to known color rendering devices to correct for color rendering errors.
2. Description of the Related Art
In the field of color rendering systems, it is commonly necessary to calibrate color rendering devices such as printers to make print colors output by the color rendering devices conform to standards and to compensate for drift and other changes. Thus, calibration is required to fine tune the color response of the color rendering device.
Printing devices typically apply three or four colorants such as cyan, magenta, yellow, and black inks. For printing both reflective prints and transparencies, the density of colorants is directly related to the quantity of ink laid down. The acronym CMYK refers to cyan, magenta, yellow, and black inks that are typically used in printing reflective prints. Cyan, magenta, and yellow are also the terms used to describe three subtractive primaries. Because inks and dyes combine in a complicated way, CMY colorants behave differently from and approximately like the true subtractive primaries: cyan, magenta, and yellow. CMY is subsequently used herein to denote colorant values.
Various printers render color differently because the colorants that they use mix differently and have different spectral characteristics. In addition, colors vary between individual printers of the same type, and the colors produced by an individual printer vary with time.
A scanner is a device for converting pictures, artwork, documents, transparencies, and photographs into electronic form such as digital image data. The scanner captures an image by measuring colors reflected from or transmitted through an image at many points (or small areas) and assigning numerical values to the colors at these points. It is common in the art to use an RGB representation scheme for electronic image colors measured by a color scanner although some scanners subsequently convert the RGB values to CMYK values. A typical digital image comprises picture elements, also called pixels that are arranged into rows and columns. These pixels together make up the image as stored in digital form and as displayed on a visual display. Each pixel of the digital image contains, among other information, the color information for that particular pixel. In the RGB scheme, the color information of each pixel is defined as having some quantity of each of the additive primary colors red (R), green (G), and blue (B). Scanners measure the color at each area, representing a pixel, of a print or transparency. Such scanners typically output RGB values.
Appearance values in appearance variable color spaces are defined as values produced by any reversible transformation of RGB. Appearance values include R, G, and B values. Other representation schemes that use appearance variable color spaces include the HSB scheme, the subtractive primary (CMY) scheme and colorimetric schemes. In the HSB scheme, the color information of each pixel is defined in quantities of hue, saturation, and brightness. The HSB appearance variables are hue, saturation, and brightness or brilliance, wherein the color of each pixel is represented by a point in the HSB color space. In the subtractive primary (CMY) scheme, the color information of each pixel is defined by the amount of the three subtractive primaries cyan (c), magenta (m), and yellow (y), which are typically not colorant amounts, as stated above. Colorimetric schemes for specifying color include use of the mathematical spaces CIELAB, CIELUV, CIEXYZ and xyY.
Prior art techniques for calibrating a color measuring device include a calorimeter for measuring the CIE values of color on a page, for example in CIEXYZ or CIELAB units. The measured CIE values are compared with a corresponding scale of desired values to obtain calibration curves.
Other prior art systems for performing printer calibration include a scanner for scanning a target. A disadvantage of using a scanner instead of a colorimeter, however, is that scanners typically use filters having spectral responses that are not optimized for measuring colorants as used in printing. Another disadvantage is that scanners operate on a sensitivity (linear) scale, not a density (logarithmic) scale. A further disadvantage is that scanners have small apertures leading to low signal-to-noise ratio (SNR). Yet another disadvantage is that the tonal and spectral responses of scanners are not standardized and thus vary from one scanner to another. Thus, different scanners do not necessarily produce the same appearance values for a spectrally identical measured color. Even further, the dynamic input range of a typical desktop scanner is generally smaller than the output dynamic range of a printer. Consequently, scanners may not accommodate measuring the entire range of ink densities that printers are able to produce. Yet another disadvantage is that the inherent resolution of the printing device, for example, a color laser printer, that generates color samples to be measured by a scanner may be close or identical to the inherent resolution of the scanner used to read the color samples. Consequently, a resolution conflict occurs that commonly manifests itself as interference patterns in the scanner signals of the image.